Concrete Products

JAN 2019

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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70 • January 2019 TECHNICAL TALK BY SABRINA GARBER Appropriate temperatures and temperature ranges are key to maintaining mass concrete durability and longevity. The risk of tem- perature-related damage increases if the temperature of concrete gets too high or the temperature differential between the hottest interior point and the surface and/or edge gets too large. Therefore, many agencies include specifications that require early-age concrete temperature not exceed some maximum internal temperature limit and that temperature differentials in early-age concrete stay below some maximum limit. MASS DEFINED The American Concrete Institute offers a simple definition of mass concrete. According to ACI 116R, it is "any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change, to minimize cracking." How large is large enough? Specifications vary among different agencies; mass concrete is defined by most specifications based on element dimensions. Structural concrete with dimension greater than 3 feet is a commonly used guideline for determining mass concrete. The dimension for classifying mass concrete ranges from 1.5 feet to 6.5 feet. Keep in mind that the ACI specification refers to dimension only. The content of the cementitious mixture used in the mass concrete will also have an impact on temperatures within those placements. Common examples of structures often categorized as mass placements of concrete include bridge piers, footings, girders, mat or thick slabs, and structural columns. Why is mass concrete temperature monitoring so important? Mass placement temperature monitoring ensures you stay within specifi- cations and reduces the risk of concrete developing severe cracking that could result in costly repairs or reconstruction. Heat generation occurs as the cementitious material in a concrete mixture hydrates. The majority of heat is generated in the first days following place- ment. This heat is highest at the center of the structure and dissipates out through the sides as it tries to reach an equilibrium temperature with the ambient conditions around it. In mass placements, because there is so much concrete flowing and cement hydrating at once, a lot of heat is generated and released, creating two primary concerns: 1) Exceeding maximum internal temperatures; and, 2) Exceeding a maximum temperature differential between the hotter and cooler part of the element. Maximum internal concrete temperature. Because heat dis- sipates more slowly in thicker placements than thinner ones, mass placements can get very hot. When internal concrete temperatures get high, there is an increased potential for Delayed Ettringite Forma- tion (DEF), a materials-related issue that can lead to severe cracking. Mass concrete specification success hinges on temperature monitoring Mitchell River Drawbridge footings PHOTOS: Transtec Group

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